In recent years, there have been proposed a wide variety of shift range switching apparatuses which are used in a conventional automatic transmission. A representative one of the known shift range switching apparatuses is called a shift-by-wire type different from a directly operated type in driving a detent lever, and comprising a sensor, or a switch for detecting the operation of a shift lever by a driver to output shift range signals indicative of the operations of the shill lever, and an actuator operated in response to the shift range signals (for example see Patent Document 1).
The shift range switching apparatus of the automatic transmission disclosed in the Patent Document 1 is provided with a shift control mechanism which comprises a shaft rotated by the actuator, a detent plate rotated in response to the rotation of the shaft, a parking rod operated in response to the rotation of the detent lever, a parking gear secured to an output shaft of the automatic transmission, a parking lock pawl for locking the parking gear, and a detent spring for restricting the rotation of the detent lever to secure the shaft range.
The shift control mechanism disclosed in the Patent Document 1 comprises a detent lever having an outer peripheral portion formed with valleys corresponding to a parking state and a parking release state, respectively, and a detent spring having a roller supported thereon. The roller is moved along the outer peripheral surface of the detent lever in response to the rotation of the detent lever caused by the rotation of the actuator to take either one of the respective positions in the valleys, so that the shift control mechanism can be switched to either one of the parking state and the parking release state. This construction leads to the fact that the shift range switching apparatus of the automatic transmission disclosed in the Patent Document 1 can be switched into at least the parking range and the shift ranges other than the parking range.
Here, the shift range switching apparatus disclosed in the Patent Document 1 is adapted to set a target rotation position immediately before the roller collides with a wall forming either one of the valleys, and to control the rotation of the actuator to have the roller stopped at the target rotation position, thereby lessening the load on the shift control mechanism.
The target rotation position is a target position acquired by the rotation of the actuator at the shift range switching time, and thus is defined with a margin in consideration of backlashes from the actuator to the detent lever with respect to a reference position where the roller is in engagement with the wall. This means that the reference position not accurately detected makes it impossible to adequately define the target position. The reference position not accurately detected results in the heavy load on the shift control mechanism with the rotation action of the actuator, thereby leading to lowering the durability of the shift control mechanism.
For this reason, the shift range switching apparatus of the automatic transmission disclosed in the Patent Document 1 is adapted to accurately detect the reference position, and thus comprises a roller pressingly engaged with a wall formed on the detent lever until the rotation force of the actuator, the restoring force of the detent spring, and the restoring force of the rod are balanced. The reference position of the actuator is detected in accordance with the relationships of the applied voltage of the actuator, and the amounts of deflection or elongation of the detent lever.